This came ahead of Friday prayers that have become a rallying point for protesters in the country's now eight-week long uprising.

Louay Hussein, a Syrian political activist, said Assad's adviser Bouthaina Shaaban had told him in a phone call on Thursday that "definitive presidential orders have been issued not to shoot demonstrators and whoever violates this bears full responsibility," according to Reuters.

Syrian soldiers, meanwhile, rolled into flashpoint cities in tanks and set up sand barriers topped with machine guns, the Associated Press reported.

Hussein was among four opposition figures who saw Shaaban earlier this month and presented demands that included an end to violent repression of protesters and the introduction of political reform in the country, ruled by the Assad family since 1970.

"I hope we will see [no firing at demonstrators] tomorrow. I still call for non-violent form of any protest regardless of the response of the security apparatus," Hussein said.

The meetings were the first between the opposition and senior officials since demonstrations calling for political freedom and an end to corruption erupted in the southern city of Deraa on March 18.

Shaaban made a similar statement to the one on Thursday at the beginning of the demonstrations in March.

Authorities have since blamed most of the violence on "armed terrorist groups backed by Islamists and foreign agitators".

Broadening the crackdown

Friday - the main congregated day of prayer for Muslims - offers the only chance for Syrians to assemble in large numbers, making it easier to hold demonstrations. This Friday will be an important test after the government said it had largely put down the unrest.

The Geneva-based International Commission of Jurists said troops had killed 700 people, rounded up thousands and indiscriminately shelled towns during the protests.

Tanks advanced in the southern towns of Dael, Tafas, Jassem and al-Harah on Thursday, broadening a crackdown before Friday.

In Deraa, a witness, who declined to be named, said the first significant demonstration since tanks shelled the city's old quarter into submission two weeks ago, erupted on Thursday.

Ammar Qurabi, head of Syria's National Organisation for Human Rights, said 13 people had been killed in the southern village of al-Harah.

Tanks also shelled a residential district in Homs killing at least five people, a rights campaigner in the city said. A sixth person was killed by a sniper.

Washington and its European allies have been criticised for a tepid response to the violence in Syria, in contrast with Libya where they are carrying out a bombing campaign they say will not end until leader Muammar Gaddafi is driven from power.

Hillary Clinton, the US Secretary of State, said Washington and its allies would hold Assad's government to account for "brutal reprisals" against protesters and might tighten sanctions, but she stopped short of saying Assad should leave power.